During Fox News' head-to-head debate between the Republican hopefuls on Thursday night, the Florida senator repeated his recent line about Bernie Sanders, the left-wing Democrat contender.
“Bernie Sanders is a socialist. I think Bernie Sanders is a good candidate for president – of Sweden,” he said, to laughter from the audience.
Only, of course, Sanders wouldn't be a good candidate for president, because Sweden's head of state is a king, with no need to undergo the indignity of elections.
Roger Lundgren, editor of Sweden's Kungliga (Royal) magazine, arguably the Scandinavian country's leading commentator on Royal affairs, questioned whether Rubio was suitable to lead his country if he was ignorant of such a simple fact about Sweden's constitution.
“One would expect that these guys, who want to be the most important leader in the world, if they want that job, they should be able to know certain facts – for example that some European countries are monarchies,” he complained.
“The thing with some American politicians, such as Sarah Palin, is – I don't want to use the word stupid, but I do. They are. They are so ignorant about the rest of the world. They think there are two monarchies in the world. And that's the UK and Monaco, because of Grace Kelly,” continued Lundgren.
"They think that every country in the world who is an ally has the same constitution as them, but we don't. It's ridiculous you know."
Dick Harrison, author of one of the most popular histories of Sweden, said that Rubio's error was not unusual among Americans.
“It's a common misconception that Sweden is a very socialist country with presidents. Actually, we have long royal history going back at least to the Viking Age, and the vast majority of the Swedish population today are in favour of a continued monarchy.”
He noted that Americans tended to have very limited understanding of Scandinavia.
“It reflects the kind of lack of knowledge about northern Europe that is common among Americans and I'm not the least bit surprised. I'm surprised that he used Sweden as an example, but not that he thinks Sweden is a republic."
Sweden is ruled by King Carl XVI Gustaf, whose ancestor Jean Bernadotte (Charles XIV John of Sweden) was a French Marshal who formed part of Napoleon Bonaparte's army.
Ann-Christine Jernberg, spokesperson for Sweden's Royal Court, said that the king would not be willing to comment on Rubio's error.
“I have no comment on that,” she said.